1. Field of the Invention
This invention relates to frequency chirped interferometric displacement sensors and mechanical translation tips therefor. Accordingly, it is a general object of this invention to provide new and improved sensors and tips of such character.
2. General Background
As indicated in the related application, cited above, classical (or phase) interferometry is well known as a means of measuring distances that are less than or on the order of the wavelength of light. Surfaces with deviations several times the wavelength of light (.lambda.), or with discontinuities greater than .lambda., cannot be measured without ambiguities of 2 .pi. in phase. Object translations greater than .lambda. can be measured by counting fringes or the number of cycles of a sinusoidal signal of fixed frequency.
In known prior art interferometric distance or profile measuring systems, the information pertaining to the optical path difference (OPD) between arms is contained in the phase of the interference pattern. There are data acquisition techniques, such as heterodyne (sometimes called phase shifting) interferometry-see U.S. Pat. No. 4,225,240 to Balasubramanian, "Method and System for Determining Interferometric Optical Path Length Difference", issued Sept. 30, 1980--in which a phase shifting or frequency shifting element is placed in one arm of the interferometer. See, also, Crane, R., Interference Phase Measurements, Applied Optics 538 (1969) and Shagan, R. N. and J. C. Wyant, Optical Frequency Shifter for Heterodyne Interferometers Using Multiple Rotating Polarization Retarders, Applied Optics 3034 (1978).
The foregoing are essentially fringe scanning interferometers in which the radiance pattern is modulated at a fixed frequency. The information is contained as a phase of that beat (fixed) frequency, and they are still beset by 2 .pi. ambiguities. A second technique used to extend interferometric measurement to larger OPD's is moire interferometry in which two interferograms are overlayed (or moireed) to form a third interferogram with a larger equivalent wavelength, as suggested by Moore, D. T. and B. E. Truax, Phase-locked Moire Fringe Analysis for Automated Contouring of Diffuse Surfaces, Applied Optics 18, 91, (1979). Moire interferometry tends to be a complicated, expensive, and difficult to implement in real time method that has found limited usefulness.
In the frequency chirped interferometer, the frequency (wavelength) of the source is modulated (chirped), resulting in fringes whose frequency rather than phase is proportional to the OPD between arms. Thus, since there are no 2 .pi. ambiguities in phase, a frequency chirped interferometer is not limited to measuring distances of less than the wavelength of light, or to translations. Phase interferometry has fractional wavelength sensitivity with dynamic range (OPD's) on the order of wavelengths. Frequency chirped interferometers have a different scale; their dynamic range is on the order of meters and their resolutions are on the order of microns. Further, because there is an internal reference (the chirp rate), they measure absolute distance rather than comparative distance.
U.S. Pat. No. 2,418,786, issued to Nadig et al. on Apr. 8, 1947, relates to a hydraulic interferometer. It discloses an interferometer having a light source, an eye-piece, and means for reflecting the light from the source to the eye-piece in the form of interfering light beams. Its reflecting means includes a vertically movable horizontal mirror. A hydraulic system of the interferometer includes connected cylinders of substantially different cross-section, a piston in one cylinder carrying the movable mirror and a plunger adjustable in the other cylinder for effecting the movement of the piston.
U.S. Pat. No. 3,374,704 to Granger, issued Mar. 26, 1968, U.S. Pat. No. 3,656,853 to Bagley et al., issued Apr. 18, 1972; U.S. Pat. No. 3,744,909 to Bruce, issued July 10, 1973; U.S. Pat. No. 4,153,370 to Corey, III, issued on May 8, 1979; U.S. Pat. No. 4,347,441 to Dil et al., issued Aug. 31, 1982; U.S. Pat. No. 4,420,260 to Martinelli, issued Dec. 13, 1983; and U.S. Pat. No. 4,475,812 to Buczek et al., issued Oct. 9, 1984 all relate to interferometers.
It appears that all of the cited patents set forth above, though possibly relating to phase directed interferometers, do not suggest usage in frequency chirped interferometers.